When you say clean water what are you referring to Distilled or Ro water ?
I have well water and
I use a triple filter filtration system
sediment ,iron , and a carbon filter and my ppm are still 330 ppm after the filters I also have a ro filter system and even then my ppm are around 30 or lower
I also use a water softener for my home
But water used to feed plants is taken before the softener FYI softened water is NG for plants
@GFDuke is correct the tds meter does not need to be calibrated
They do sell test fluid that’s a know ppm you can test your meter with to ensure it’s reading true you can find it on Amazon